Genome-wide identification and transcript analysis during fruit ripening of ACS gene family in sweet orange (Citrus sinensis)

Abstract

Ethylene plays an important role in regulating the development and ripening of fruits, and ACS (1-aminocyclopropane-1-carboxylicacid synthase) is the key rate-limiting enzyme in ethylene biosynthesis pathway. However, identification and functional analysis of ACS gene family in non-climacteric citrus have never been reported. In the present study, we identified eighteen probable CsACS genes from the genome of sweet orange (Citrus sinensis cv. Valencia). Subsequently, the phylogenetic, structural, promoter, microsynteny and expression analysis were performed. The CsACS genes mainly located outside the cells, and were unevenly distributed on 8 chromosomes of sweet orange. Phylogenetic relationship analysis showed that eighteen CsACSs were divided into four groups, including three types of ACS, and the ACS genes with similar gene structure and motif distribution were clustered in the same group. Cis-element identification indicated the possible role of CsACS genes in growth and development processes. Conserved synteny was observed among C. sinensis, Arabidopsis and Solanum lycopersicum, and orthologous gene pairs were distributed across the eight chromosomes with ACS genes of C. sinensis. Furthermore, seven CsACSs (CsACS1 to CsACS7) tissue-specific expression results showed that five of the seven members were highly expressed in the ovary. In conclusion, based on CsACS genes expression level, changes in ACS activity and ACC content at different stages of sweet orange fruit, the CsACS1 and CsACS2 may regulate ethylene synthesis at the early stage before coloring, whereas stable and low-level endogenous ethylene synthesis during coloring and ripening stages may be regulated by the expression of CsACS3, CsACS4 and CsACS6. Taken together, these results would provide valuable information for future functional investigation and application for breeding of ACS gene family in sweet orange.